This is a study of the structural and electronic behavior and properties of graphene on α-quartz and α-sapphire using Density Functional Theory. We construct initial structures using the above 2 substrates, place a layer of graphene on them and subsequently allow the atoms to relax. After relaxation we study any structural changes, band structures, density of states, charge density to determine the electronic properties of the entire structure. Eventually this study will help in the search for good substrates for graphene based transistors. / text

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In this thesis first-principles calculations of point and extended defects in diamond and silicon are reported. In single crystal diamond grown by chemical vapour deposition (CVD) dislocations are observed as mixed-type 45° and edge-type dislocations lying along <100> with 1/2<110> Burgers vectors. Results are presented on the core structures, core energies and electrical properties of both types of dislocations and their interaction with nitrogen is investigated. Then the focus turns to the brown diamond problem. Despite concerted research efforts, the origin of the brown colouration of diamond is still under discussion. Recently, the attention was drawn to vacancy-related defects. Experiments on type IIa diamonds indicate that the brown colour is caused by vacancy-type extended defects, however the shape and size of these defects remained unclear. In this work, the structural, electrical and optical properties of large spherical vacancy clusters and thin vacancy disks are investigated by means of density functional theory and the calculations are compared with recent experimental measurements on brown diamond. High pressure high temperature treatment (HPHT) of brown type Ia diamonds above 2000°C results in the loss of the brown colour and the formation of nitrogen-vacancy defects. The generation of such defects requires a source of mobile vacancies during the annealing process. It is suggested that the vacancy cluster model described in this thesis can explain the observed annealing behaviour since the break-up of the clusters leads to a supersaturation of mobile vacancies which readily complex with substitutional nitrogen atoms present in the material. Therefore, the effect of HPHT treatment of brown type Ia diamond is investigated by studying the formation energies of common and rare defects and estimates of their equilibrium concentrations at different annealing stages are given. Finally, an open problem also involving nitrogen, but in a different group IV semiconductor is considered. In Czochralski-silicon, nitrogen-related shallow thermal donors are formed between 500 and 750°C. Until now the exact chemical composition and atomic structure of these defects are not well established. Here, it is shown that NO and NO_2 belong to the family of nitrogen-oxygen related shallow thermal donors. Based on the law of mass action the equilibrium defect concentrations are predicted. Finally, the theoretical results are compared to recent Fourier transform infrared (FTIR) spectroscopy measurements.

The properties of a new family of carbon structures based on stacked cyclic C6 rings and intercalated cyclic C6 structures: (C6)n and (C6)nMen-1 have been studied theoretically using ab initio DFT (Density Functional Theory). Calculations of the structural, electronic, and vibrational properties of a range of these molecules have been carried out using DFT techniques with the best correspondence to experimental results. The chemical and structural stability of structures based on stacks of cyclic C6 has also been estimated for pure carbon molecules (C6)n and for metal-organic sandwich molecules intercalated with Fe and Ru atoms. These have (C6)nFen-1 and (C6)n Run-1 compositions, respectively These structures are predicted to show a variety of new electronic, vibrational and magnetic properties. Ultra-small diameter tubular molecules are also found to have unique rotational electron states and high atomic orbital pi-sigma hybridization giving rise to a high density of electron states. All phonons in these structures have collinear wave vectors leading to an ultrahigh density of phonon states in dominant modes suggesting that some of these structures may exhibit superconductivity. These properties, as well as a predicted high electron mobility, make these structures promising as components in nanoelectronics. Experiments using femto-second laser pulses for the irradiation of organic liquids suggest that such structures may appear under certain conditions. In particular, a new type of iron carbide has been found in these experiments.

Aluminum complexes, [O3PMe]AlR(R = OtBu, OPh), containing tris-(3,5-di-tert-butyl-2-hydroxy-phenyl)phosphine ([O3P]H3) which is a novel tridentate ligand have been synthesized and characterized by NMR, X-ray diffraction and elemental analysis. Theses complexes were used as catalysts for ring-opening polymerization of £`-caprolactone. We suggested that the stereo effect of catalysts is the main factor in the ring-opening polymerization and compared the mechanism with DFT. In additional, we studied the electronic states and electronic chemistry of [O3PMe]AlR by DFT, UV and PLE. The novel ligand, bis(3,5-tert-butyl-2-phenoxy) tert-butylphosphine ([tBuOPO]H2), reacted with alkali metals such as n-BuLi, NaH and KH to form a series metal complexes, [tBuOPO]M2(Solvent)x (M = Li, Na, K). These metal complexes are all dimeric molecules characterized by X-ray diffraction, NMR and elemental analysis. Moreover, we reacted {[tBuOPO]Li2(DME)}2 with metal complexes of group 4, TiCl3 and MCl4(THF)2 (M = Ti, Zr, Hf), and we received [tBuOPO]2M and [tBuOPO]MCl2(THF) (M = Ti, Zr, Hf). We also synthesized alkoxide complexes of the series metal complexes and studied the catalytic reactivity for ring-opening polymerizations. Furthermore, tantalum complexes, [tBuOPO]2TaX (X = F, Cl) and [tBuOPO]TaCl3, have been synthesized and characterized. Especially synthesizing [tBuOPO]TaCl3 should be carefully controlled by lowering the concentration of TaCl5.

The atomic and electronic structures of Au-intercalated graphene buffer layer on SiC(0001) surface were investigated using first-principles calculations. The unique Dirac cone of the graphene near K point reappeared as the buffer layer was intercalated by Au atoms. Coherence interfaces were used to study the mismatch and strain at the interfaces. Our calculations showed that the strain at graphene/Au and Au/SiC(0001) interfaces also played a key role in the electronic structures. Futhermore, we found that at Au coverage of 3/8 ML, Au intercalation leads to strong n-type doping of graphene. At 9/8 ML, it exhibited weak p-type doping, meaning that graphene is not fully decoupled from substrate. The shift of Dirac point resulting from electronic doping is not only due to different electronegativities but also strains at the interfaces. Our calculated positions of Dirac points are consistent with those observed in the ARPES experiment [Isabella Gierz et al., Phys. Rev. B 81, 235408 (2010).].

The behavior of dense colloidal fluids near surfaces can now be probed in great detail with experimental techniques like video and confocal microscopy. In fact we are approaching a point where quantitative comparisons of experiments with particle-level theory, such as classical density functional theory (DFT), are appropriate. In a forward sense, we may use a known surface potential to predict a particle density distribution function from DFT; in an inverse sense, we may use an experimentally measured particle density distribution function to predict the underlying surface potential from DFT. In this dissertation, we tested the ability of the closure-based DFT to perform forward and inverse calculations on potential models commonly employed for colloidal particles and surface under different surface topographies. To reduce sources of uncertainty in this initial study, Monte Carlo simulation results played the role of experimental data. The accuracy of the predictions depended on the bulk particle density, potential well depth and the choice of DFT closure relationships. For a reasonable range of choices of the density, temperature, potential parameters, and surface features, the inversion procedure yielded particle-surface potentials to an accuracy on the order of 0.1 kBT. Our results demonstrated that DFT is a valuable numerical tool for microcopy experiments to image three-dimensional surface energetic landscape accurately and rapidly. B

In order to understand the structural and electronic properties of titanium oxide nanoparticles of different sizes, the FIRE algorithm combining the simulated annealing method is employed to find the structures of TinO2n (n=1¡Ð6) nanoparticles with the global minimum potential energy. To deeply understand electronic properties, the relaxation structures of TinO2n (n=1¡Ð6) nanoparticle from previous method will be recalculated by density functional theory (DFT) method. The Fukui function, Frontier Molecular Orbital and density of state of TinO2n (n=1¡Ð6) nanoparticles are discussed for understanding the size effect of TiO2 nanoparticles on chemical reactivity. The adsorption and dissociation energy mechanism of the HN3 molecule and its fragments are also discussed and are compared with the mechanism about HN3 on the anatase surface.